1. Field of Invention
The invention is related to a subcutaneously implanted vascular access device including a main body having a tapered, fluid-filled cavity. The invention is also related to a vascular access device including a percutaneously placed catheter.
2. Related Art
End stage renal failure patients require repeated, chronic access to their vascular system to allow life sustaining hemodialysis. In the United States, an estimated 250,000 patients per year undergo hemodialysis and, thus, require maintenance of a vascular access site. The estimated cost associated with this procedure is approximately $8000 dollars per year per person. Cumulatively, 2 billion US dollars/year are spent.
Current vascular access systems have limitations that commonly lead to system failure requiring interventions to reestablish access. There are three commonly-used systems to establish vascular access for kidney failure patients.
Catheter-based access. This system may include a synthetic catheter that may be placed transcutaneously. Alternatively, the catheter may have an attached buried reservoir that allows the dialysis machine operator to attach the patient's catheter to the hemodialysis machine. Although this system offers immediate availability for use after placement, there may be several disadvantages. First, the system may be prone to infection, particularly for transcutaneously placed catheters where infection can develop at the skin exit site. The presence of a foreign body in the vascular system is prone to seeding from inadvertent “breaks in sterile technique” as technicians repeatedly access the system to place patients on the dialysis machine as well as seeding from distant sources of infection in the patient via the bloodstream. When a catheter becomes infected, removal is usually required to clear the infection. As a result, another catheter must be placed at another site to resume treatment. This requirement results in the additional cost and inconvenience of performing an additional surgical procedure as well as the cost of the replacement catheter.
Second, the relatively small caliber of the synthetic tubes placed in the vascular system may permit only limited blood flow through the system while a patient is on the dialysis machine. This may prolong the time each patient needs to be dialyzed and may indirectly contribute to the frequency of dialysis treatments.
Third, the catheter system introduces a foreign body in a central vein. This can lead to further complications including, for example, frequent formation of fibrin clots or blood clots in the lumen of the tube requiring the infusion of costly thrombolytic drugs to clear the tubes. Sometimes even more costly endovascular procedures are required to reestablish function in a catheter. Furthermore, the foreign body tube in the vascular system may result in a thrombosis or a narrowing (stenosis) of a major vein (subclavian vein or superior vena cava) that then precludes using that vein for future venous access in that patient. This complication can have life threatening ramifications particularly if the stenosis involves the superior vena cava.
Native Arterial Venous Fistula. A second mode of vascular access requires the construction of a connection or arterial venous (AV) fistula between a patient's native vein and an adjacent artery that creates a high flow of blood through a limited portion (generally 15-20 cm) of the patient's vascular system. This superficially positioned natural vascular conduit usually grows to a certain minimal size (approximately 5-6 mm or greater in diameter) that then provides a target for the dialysis technician to insert the two needles that are required to place a patient on the dialysis machine. Some advantages of a natural AV fistula may include, for example, longevity, immunity from infection, and low cost. This type of fistula has the longest life span of all known types of vascular access, lasting from several years up to twenty years or more, with the possibility of performing a secondary surgical intervention to salvage the functioning of the fistula even if complications do develop with the fistula over time. Also, since the fistula is constructed of “native tissue,” it is relatively immune to infection. Moreover, the fistula is cheaper to construct since there is no requirement for a costly catheter or synthetic graft.
On the other hand, since there are limited sites for the creation of a functioning fistula in each patient, the current standard of care is to perform the fistula in the most distal vessels in a patient's upper extremity where a preoperative assessment indicates that the fistula has a reasonable chance for maturing successfully. A 20% failure rate is considered acceptable for “first time” fistulas as vascular access surgeons try to maximize the available sites in a patient cognizant that patients may require new fistulas at other sites in the future if the primary fistula eventually fails. This 20% failure rate can lead to further operations and, thus, significant additional costs and inconvenience to the patient.
Access to a fistula is highly dependent on the skill of the dialysis technician. A fistula that, at maturation, is somewhat smaller than desirable, or is located deeper in the patient's tissues presenting a less easily palpable target than normal, is more likely to be damaged during attempts to access the fistula. Native fistulas can be damaged in a variety of ways. Repeated access through nearby adjacent sites can lead to localized trauma to the wall of the fistula causing a weakening or “ballooning out” of the wall of the fistula. This local aneurysm can lead to a failure of the fistula if not repaired. Also, if the technician inadvertently punctures the “back wall” of the fistula while trying to gain access through the “front wall,” or if inadequate pressure is held over a needle access site at the end of the dialysis run, a local collection of blood (called a hematoma) can form that, at the least, can prevent access to the fistula over the next several weeks in that same location. At worst, the blood collection can progress to form an organized scar that constricts the fistula, possibly leading to its ultimate failure.
In the United States, the preferred method that is taught to access native fistulas is the step ladder approach, i.e., constantly moving the locations where the two access needles are placed in the fistula. The protocol of sticking needles into the fistula at different locations at each dialysis session leads to increased pain experienced by the patient since the effect of a localized area of insensitive scar tissue—e.g., where the fistula is accessed using the “buttonhole” technique (discussed further below)—is never allowed to form. Moreover, the native fistula may require at least 4 weeks and sometimes up to several months to “mature,” i.e., to grow to an adequate size and increased thickness of its walls that will allow the vessel to be safely punctured with a needle.
The “buttonhole” technique to access fistulas, favored in Europe, has been shown in limited studies to increase the longevity of a fistula while decreasing complications associated with moving access sites to different locations in the fistula during subsequent dialysis sessions. Some advantages of the “buttonhole” technique may include: (a) two nearby needle puncture points to access the fistula (approximately 3 cm apart) require only a relatively short functioning and accessible fistula to access the bloodstream; (b) the scar tissue that develops from repeated puncture of the skin in two locations causes a relative insensitivity of the skin to puncture pain; (c) the narrow cicatricial tract self seals relatively easily with thrombus after removal of the needles following a dialysis run, decreasing the incidence of perifistula hematomas that can temporarily, or even permanently, incapacitate a fistula; and (d) after a cicatricial track has been established, “blunt” needles can be used to access the graft, decreasing the incidence of needle point damage to the back wall of the fistula when inserting needles.
A disadvantage of the “buttonhole” approach is the logistical problem that requires a highly skilled dialysis technician to create a “button-hole” track. Ideally, a single highly-skilled technician will repetitively perform the needle insertion on the same patient during the initial dialysis sessions, following the exact needle track in the same patient (same entrance point, same angle, same depth) every time for the first 10-15 access events until a well formed tract has developed. The logistical difficulty of having the same highly-skilled technician available for the first 10-15 access events in an individual patient currently limits the wider applicability of this technique.
Synthetic Bridge Graft Fistula. A third mode of vascular access requires the placement of a synthetic graft in a subcutaneous position, usually in a patient's upper extremity. The technician achieves access to the vascular system by placing needles directly into the easily palpable graft. Some advantages may include that the graft provides a reliable, easily accessible conduit to access to connect a patient to the dialysis machine. This choice of access is particularly valuable in patients who do not have the requisite minimally sized vein that will permit the establishment of a “native” AV fistula.
On the other hand, the cost of such a graft is approximately $500 (US) per patient. Also, the most commonly placed grafts require several days and up to several weeks for perioperative swelling to decrease and for the grafts to become sufficiently incorporated into a patient's tissues to allow safe access via the graft. The graft can also become infected by inadvertent “lapses of sterile technique” by the dialysis technician or through seeding from distant sources in the patient. An infected graft frequently requires a very costly and inconvenient (to the patient) series of procedures that includes removal of the infected graft, placement of an interim dialysis catheter, and a subsequent implantation of a new graft after the infection has been definitively treated to reestablish vascular access. A synthetic graft can also be prone to development of early or late thrombosis—due its synthetic “foreign” quality.
Moreover, the site of the venous anastomosis between the graft and the patient's native vein can be a frequent site of stenosis that develops from a mismatch in the distensibility characteristics of the patient's native vein and the synthetic graft. This can require secondary costly surgical or endovascular interventions to correct the problem and to preserve continued functioning of the graft as a viable access conduit. Some grafts may even form a stenosis within the graft due to a proliferation of fibrin and scar tissue that requires a secondary procedure to maintain the viability of the graft. Additionally, repeated puncture of a graft in the same location by the dialysis technician can lead to a “pseudoaneurysm” formation (i.e., a localized collection of blood) that can eventually lead to failure of the graft if not corrected. Improper technique by the dialysis technician can also result in a hematoma formation during access to the graft or following removal of the needles at the end of the dialysis run. The hematoma may make access to the graft in that location difficult or impossible for a period of time, and may lead to occlusion of the graft.
Vascular access devices are needed that substantially overcome the foregoing disadvantages.